1. Field of Invention
The present invention relates to a coupon, generally used for monitoring conditions that could lead to corrosion or stress corrosion cracking under a disbanded coating on a metallic surface. More particularly, the present invention relates to a steel coupon, preferably in the form of a pipe segment, on the outer diameter of which various types of coating disbondment geometries anticipated on a pipeline are fabricated, and in which the environment of the coating disbondment geometries can be continuously or periodically monitored and/or measured.
2. Description of Related Art
Metallic surfaces, such as are common in the field of pipelines and related structures, such as buried tanks and distribution systems, are adversely affected by numerous corrosive electrolytic fluids that contact these surfaces. For example, in the natural gas and petroleum industries, corrosion occurs extensively on the outer surface of both buried and above-ground pipelines.
In order to reduce, or preferably entirely eliminate, this undesirable metallic surface corrosion, anti-corrosion protective coatings have been extensively used in the pipeline industry. These ubiquitous anti-corrosion protective coatings frequently take the form of a helically-applied tape-like protective outerwrapping. The tape-like protective component may be applied directly over an unprepared pipeline outer surface, or may, in fact, be overlaid onto a primer-coated, pretreated pipeline outer surface. Other forms of protective coatings also exist, including coal tar epoxy, asphalt, and fusion bonded epoxy coatings.
For example, U.S. Pat. No. 5,391,686 discloses an exemplary pipeline coating material. The patent discloses a polyurethane formulation that is described to be especially effective as a corrosion preventing coating for metallic substrates. The polyurethane is prepared by mixing two components, one being a mixture of high and low molecular weight polyol compounds and the other being a polymeric methylene polyphenyl isocyanate (MDI).
However, despite the use of such pipeline coating compositions and materials, external corrosion of pipelines are a concern, when there is a breach of the coating, called a xe2x80x9choliday.xe2x80x9d
To protect against external corrosion, pipelines (and other buried structures) are typically cathodically protected in addition to being coated as described above. The cathodic protection (hereafter alternatively referred to as xe2x80x9cCPxe2x80x9d) system is designed to protect the pipe where coating defects occur. Cathodic protection, as it is used here, refers to the phenomenon and practice of applying a small potential to a metallic pipeline that is buried in the ground. This imparted cathodic potential of the buried pipeline will tend to limit or protect against corrosion attacking the metal surface.
Cathodic protection provides corrosion protection to any bare metal areas exposed to soil due to coating defects or xe2x80x9cholidaysxe2x80x9d by causing direct current to flow from the soil into the structure, thereby polarizing the structure as a cathode. Protection is ensured by modifying the environment around the steel as well as reducing the dissolution rate of the steel by reducing the anodic overpotential. The required direct current output of the cathodic protection system is reduced to manageable levels by the coating, which substantially reduces the bare metal area of the structure exposed to soil.
Two cathodic protection systems are generally in use for corrosion protection of metal structures. The first, termed an impressed current cathodic protection system, consists of a rectifier, insulated wires connecting the plus terminal of the rectifier to a buried anode (for instance graphite cylinders), insulated wire connecting the negative terminal of the rectifier to the protected structure, and test stations installed at the structure. The test stations typically consist of a pipe or a valve box with one or two insulated wires attached to the structure, typically by brazing, and a terminal board for termination of the wires. The test stations are used for monitoring the corrosion protection levels by measuring potentials between the structure and a reference electrode in an electrical contact with ground above the structure. The reference electrode usually consists of a copper rod fixed in a plastic body filled with saturated copper-sulfate solution, and having a porous plug to facilitate electrical contact with the ground.
The second, termed a sacrificial (galvanic) cathodic protection system, consists of magnesium, zinc or aluminum anodes buried next to the structure and often directly connected by an insulated wire to the structure. The protective current is generated by the potential difference between the structure and the anode. The structure with sacrificial anodes also has test stations for the cathodic protection testing and evaluation of its corrosion protection effectiveness.
Details of different cathodic protection systems and of the pipeline potential measurements can be found in W. von Baeckmann et al., Handbook of Cathodic Corrosion Protection, Theory and Practice of Electrochemical Protection Processes 3rd Ed., Houston, Tex.: Gulf Publishing Co., 1997, the entire disclosure of which is incorporated herein by reference.
Even when cathodic protection is used, corrosion and defects tend to still occur. Coating defects generally take the form of either exposed bare steel or a disbondment where the coating remains intact but a gap is formed between it and the bare steel. CP systems effectively deliver current to defects directly exposed to soil or water and therefore mitigate corrosion. However, defects covered by a disbonded coating are sometimes difficult to protect because the coating shields the CP current from reaching deep inside the crevice (i.e., disbanded region). In addition, stress corrosion cracking and localized corrosion have been observed predominantly under disbanded regions of the pipe.
To determine the level of CP required for protection (in accordance with NACE standard RP0169), voltage measurements are taken of a pipe versus a reference electrode placed on the soil surface. CP coupons are samples of steel that are electrically bonded to the pipeline and CP system and simulate bare steel coating defects exposed to the soil. Briefly disconnecting the coupon from the pipe while measuring the potential of the coupon with respect to the soil, eliminates the ohmic resistance induced potential drop errors associated with coupon potential measurement. The limitation to this design is that only one geometry is represented by the simulated defect, while pipelines have a broad spectrum of defects. The most poorly represented pipeline coating defects are those under disbonded coatings.
Various CP systems are known in the art, as are coupons for assessing and measuring the CP systems. For example, CP system coupons are disclosed in U.S. Pat. Nos. 5,814,982, 6,060,877, and 6,107,811, the entire disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,814,982 discloses a test station for measuring the effectiveness of cathodic protection. The test station includes a cylindrical plastic reference tube extending downward through the soil to near the protected structure, such as a pipe. Two plastic coupon tubes extend within the chamber of the reference tube and are attached to opposite sides of interior sidewalls of the chamber. First and second circular cylindrical rod-shaped coupons are attached to, and sealed against, the bottom ends of the coupon tubes. A first coupon is electrically connected to the pipe, and the second coupon is used to measure a free-corrosion (native) potential.
U.S. Pat. No. 6,060,877 discloses a flat cathodic protection test probe. The probe comprises a non-metallic probe body with a mounted metal coupon simulating a large coating holiday on a pipeline, an insulated wire attached to the coupon for an electrical connection to the pipeline, a non-metallic tube filled with a conductive backfill, and a narrow and long porous potential sensing strips mounted flush with the coupon exposed surface in the central area of the coupon. The coupon is electrically insulated from the conductive backfill by a non-metallic coating and an epoxy compound for electrical insulation. To determine if the pipeline is fully cathodically protected, the flat cathodic protection probe is buried next to the pipeline and interconnected by the insulated wires with the pipeline. The potentials taken on the probe will not include a substantial voltage drop in soil and will not be adversely affected by shielding of the coupon by a reference electrode or its access tube to invalidate the reading. To measure the probe potential, a calibrated reference electrode is lowered into the tube to make an electrical contact with the conductive backfill and a high impedance voltmeter is connected to the coupon and to the reference electrode. No interruption of the cathodic protection to the pipeline or coupon is necessary.
U.S. Pat. No. 6,107,811 discloses an automated coupon monitor for detecting the electrical parameters associated with a pipe, a coupon and a reference electrode utilized with a cathodic protection system. A voltage detector circuit has its output applied to a pair of sample and hold circuits, one for recording EOFF and the other for recording EON. Connected in series between the coupon and the pipe is a zero resistance current detector circuit and an interrupter switch for measuring the pipe-coupon current and periodically interrupting that current to permit measurement, sampling and storing of EOFF. The sample and hold circuits and the interrupter switch are controlled by a microprocessor controller. The circuit provides DC level outputs which can be easily read by a digital multimeter or stored in a data logger for subsequent reading or transmission to another location.
U.S. Pat. No. 5,728,943 describes the use of a plurality of mechanically stressed coupons maintained at various potentials to monitor stress corrosion cracking of pipeline. However, such a method does not recognize the fact that most stress corrosion cracking occurs under a disbanded coating and therefore does not provide a method to monitor conditions under a disbonded coating.
However, despite the above-described methods and materials for protecting a pipeline from corrosion, and for monitoring any corrosion that may occur, improvements in the pipeline industry are still desired. For example, the above-described references, while describing various coatings and CP systems, do not relate to measuring the effects of CP under disbanded coatings. Thus, the described coatings and CP systems do not provide any means to non-destructively determine the effectiveness of the coatings and systems, particularly with respect to disbonded coatings.
Unfortunately, the most susceptible location for corrosion and stress corrosion cracking on a pipeline with CP that meets standard protection criteria is underneath disbonded coatings. Successful development of a disbanded coating CP coupon would have wider application than conventional CP coupons because potential cannot be measured under disbondments for all coated pipelines (and not only for those where all sources of current cannot be disconnected).
Since most U.S. pipelines are coated and meet industry standard CP protection criteria (mandated in the United States by 49 C.F.R. Parts 192 and 195), a need exists for monitoring corrosion protection levels under disbanded coatings. No method presently exists to detect the onset of corrosion (including stress corrosion cracking) in the most vulnerable locations on pipelines, i.e., in the area of disbonded coatings.
In addition, accurate measurement of potential would allow a pipeline operator to determine if mitigation methods are effective. The simplest and least expensive mitigation is to apply more CP current by adjusting rectifier output or adding more sacrificial anodes. Sensing pH would provide an additional criterion to assess if corrosion is occurring underneath disbondments.
The only present methods to detect corrosion under disbanded coatings are inspection (e.g., smart pigs) and post failure analyses. Both of these methods find metal loss after the corrosion damage has already occurred; a disbonded coating coupon would be used to detect the onset of damage before significant metal loss occurs. A disbonded coating coupon might also provide historical information on changes under a disbondment. For example, in locations where stress corrosion cracking has been detected, the only information available is the post-failure analysis of the solution drawn from under the disbanded coating after excavating the pipe. Such an analysis may not accurately reflect the changes in the environment prior to cracking and may be affected by the excavation and extraction operations. For regulatory compliance, operators can use the tool to demonstrate the effectiveness of CP at a particular location given a range of coating disbondments.